This invention relates to drill bits for jet assisted rotary drilling and its object is more particularly related to the optimization of the geometry of drill bils.
As known, a drill bit is a mining tool which is mounted at the end of a drill pipe driven into rotation and which is used for biting in and penetrating rocks parallel to its rotary axis by means of sharp edges formed on its working face. Such sharp edges generally consist of plates of high hardness most often made of tungsten carbide and built up by brazing; it is then specified sometimes that the drill bit has built up inserts.
Such drill bits are used in practice for drilling into relatively soft and little abrasive rocks. For harder and/or more abrasive rocks it has been proposed to utilize rotary percussion drilling with impacting effect but such drilling appears to have serious disadvantages in as much as, apart from resulting in very high installation costs, it gives rise to noises, vibrations, and oil vapours, very detrimental to the ergonomical rules, or even to security.
Another solution, which appears to have great future, i.e. jet assisted rotary drilling, has however been proposed recently for drilling semi-hard rocks (pressures higher than or equal to about 800 bars), and hard and abrasive rocks.
This method consists in driving a drill bit into simple rotation without impacting effect, while injecting close to its sharp cutting edges a very high pressure fluid which fractures rocks and thereby facilitates cutting down thereof by the drill bit.
The utilization of such method gave rise to certain difficulties related to supplying said fluid under very high pressure, most often water, through the drill pipe and the drill bit. It is however to be noted that it was already known to feed water or air at low pressure (about 20 to 40 bars) to the drill bit for moving away drilling or cutting debris.
French patent No. 2 450 936 (G.CAGNIONCLE) filed on Mar. 8, 1979 describes a method for getting over such difficulties. For utilizing the method described therein it is contemplated to form through a drill bit two sets of pressurized fluid channels i.e. low pressure fluid inlet channels for removal of debris, and much narrower channels for feeding fluid under very high pressure (1000 to 4000 bars), for assisting the drilling proper, opening through or forwardly of the built up inserts. This patent also describes the whole liquid supply device required for fluid injection under two very different pressures.
It appears actually that the drill bits employed heretofore for purposes other than assisted rotary drilling have a geometry very close to that of the conventional drill bits (without jet assistance).